Sauté the veggies: Heat oil in a large skillet over medium heat. Add onion and bell pepper.
Cook 4–5 minutes until softened. Stir in garlic and cook 30 seconds until fragrant.
Season the chicken: Add cooked chicken to the skillet. Sprinkle in chili powder, cumin, smoked paprika, oregano, salt, and pepper.
Toss to coat and warm through, about 2–3 minutes.
Add brightness: Turn off the heat and stir in lime juice. Taste and adjust seasoning. Mix in cilantro or jalapeños if using.
Build the quesadillas: Place a tortilla on a plate or board.
Sprinkle a thin layer of cheese over half the tortilla. Add a layer of the chicken mixture, then top with another light layer of cheese. Fold the tortilla over to make a half-moon.
Toast to crispy perfection: Heat a clean skillet over medium to medium-low.
Add a small pat of butter or a light brush of oil. Place the folded quesadilla in the pan. Cook 2–3 minutes per side until the tortilla is golden and the cheese is melted.
Adjust heat as needed to prevent burning.
Repeat and keep warm: Transfer cooked quesadillas to a cutting board. Repeat with remaining tortillas. If making several, keep them warm in a low oven (200°F / 95°C) while you finish the batch.
Slice and serve: Cut each quesadilla into wedges.
Serve with salsa, guacamole, sour cream, and lime wedges.